The petroleum industry continues to expand deepwater exploration and drilling efforts in many areas of the world. As drilling water depth increases, the potential for natural gas hydrate formation during drilling operations also increases.
Gas hydrates are solid, ice-like crystals that form under elevated pressures and at moderately low temperatures. Gas hydrates consist of water molecules which form five (pentagon) and six (hexagon) membered polygonal structures which combine to form closed structures (often called a "cage"). These "cages" totally enclose or trap a gas molecule. At high pressures, multiple "cages" tend to combine to form larger cages enclosing gas molecules. The resulting large crystalline assemblies are thermodynamically favored at elevated pressures. Under sufficient pressure, gas hydrates will form at temperatures well above the freezing point of water.
Primary promoters of gas hydrates are gas with "free" water present at or below its water dew point, low temperatures, and high pressures. Secondary promoters are high velocities, pressure pulsations, any type of agitation, and the introduction of a small crystal of a hydrate. During deepwater drilling operations, all of the primary gas hydrate promoters are present.
The formation of gas hydrates in deepwater drilling operations creates safety concerns and can prevent proper operation of the blowout preventer during well-control operations, causing costly delays in controlling the wells. The use of a 20% sodium chloride/PHPA drilling mud system suppresses gas hydrate formation by 13.3-15.6.degree. C. (24-28.degree. F.) below that of freshwater. Often, however, it is necessary to suppress the temperature of hydrate formation even further--for example, during disconnects. "Spotting" of certain materials reportedly is effective to temporarily remove hydrates from various equipment and/or lines; however, spotting often is not performed until after gas hydrates have formed and caused the very delays sought to be avoided.
Another problem that occurs during deep water drilling is that "water-sensitive shales"--or formations having clay minerals as major constituents, such as shales, mudstones, siltstones, and claystones--often must be penetrated before reaching the hydrocarbon bearing zone. Various problems are encountered when drilling through water-sensitive shales, particularly using water-base drilling fluids. Water adsorption and hydration of the shale typically results in stress and/or volume increases, and can induce brittle or tensile failure of the formation. Such failures lead to sloughing, cave in, and stuck pipe. The volume increases also reduce the mechanical strength of the shales, and cause swelling of the wellbore, disintegration of cuttings in drilling fluid, and balling up of drilling tools.
Low density drilling fluids are needed which inherently will provide hydrate suppression under deep water drilling conditions, which will decrease water adsorption and hydration of water-sensitive shales, and which still have desirable rheology and fluid loss control properties for deepwater environments.